Method for the preparation of semiconductor cadmium compounds



United States Patent METHOD FOR THE PREPARATION OF SEMI- CONDUCTOR CADMIUM COMPOUNDS Herbert Miller, Needharn, Mass, assignor to Monsanto Chemical Company, St. Louis, Mo., a corporation of Delaware No Drawing. Filed Apr. 25, 1058, Ser. No. 730,825 14 Claims. (Cl. 2'3-50) The present invention relates to a new method for the preparation of cadmium sulfide, cadmium selenide and cadmium telluride. It is an object of the invention to prepare a photoconductive, single-crystalline form of the said cadmium compounds. It is a further object of the invention to prepare cadmium sulfide, selenide and telluride by the reaction of elemental cadmium with an alkyl compound selected from the group consisting of alkyl mercaptans, alkyl sulfides and alkyl disulfides, alkyl selenols, alkyl selenides, alkyl diselenides, alkyl tellurols, alkyl tellurides and alkyl ditellurides.

Cadmium sulfide has been recognized in the prior art as having photoconductive properties. It has been found, however, that the prior art methods which have been available for the preparation of cadmium sulfide, cadmium selenide and cadmium telluride failed to produce sufficiently pure materials to obtain reproducible photoconductive properties. The prior art has endeavored to solve this problem by providing increasingly pure sources of cadmium metal, for example by the use of the zone refining method. However, even the use of very pure cadmium, together with the prior art sources of sulfur, selenium and tellurium, respectively, have not been sufficient to insure the reproduction of a photoconductive grade of cadmium sulfide, selenide or telluride.

It has now been found that an unusually pure form of single-crystal cadmium sulfide, cadmium selenide or cadmium telluride may be obtained by the reaction of elemental cadmium metal in the vapor phase with an alkyl compound of sulfur, selenium or tellurium, respectively. Suitable alkyl sulfur compounds for this reaction include the alkyl mercaptans, alkyl sulfides, alkyl disulfides, alkyl selenols, alkyl selenides, alkyl diselenides, alkyl tellurols, alkyl tellurides, and alkyl ditellurides in which each alkyl radical has less than 4 carbon atoms. A preferred group of compounds are the alkyl sulfur, selenium and tellurium compounds having a boiling point of less than 100 C. It is also desirable that the dissociation energy of the alkyl compound which is employed be less than 90 kilo-calories/mole and the use of such group of compounds constitutes a preferred embodiment of the invention.

The present method for the production of cadmium sulfide, cadmium selenide and cadmium telluride is carried out at a temperature in the range of from 750 C. to 1,200 O, a preferred range being from 800 C. to 1,000 C. The pressure at which the reaction occurs is not critical and may be varied broadly, i.e. from the use of a moderate vacuum to medium pressures, such as 5 or as much as 50 atmospheres. The alkyl sulfur, selenium or tellurium compound is preferably introduced into the reaction system in vapor form and is there contacted with the vapor of cadmium metal. If desired, a carrier gas stream of an inert gas, preferably a reducing gas, for example, hydrogen, nitrogen, argon or helium may also be passed into the reaction zone in order to maintain a turbulent gas mixture for the reaction of the alkyl compound with the cadmium metal.

mixture of hydrogen with distilled dimethyl sulfide.

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The reaction is preferably conducted in a high temperature furnace with a quartz combustion tube that has a condenser system or cool zone for the growing of the photoconductive grade of single-crystal cadmium sulfide or other product. 'The cadmium metal may be introduced in vapor form into the reaction zone or may be introduced by a carrier gas stream. In small scale installations the cadmium metal may be placed in a crucible or boat in the electric furnace and vaporized therein for reaction with the incoming alkyl compound. The preferred reaction temperature maintained in the furnace is from 800 C. to 1,000 C. The reaction zone is adjacent to a somewhat cooler crystallization zone. The temperature in the crystal-growing zone is preferably maintained in the range of from 750 C. to 850 C. in order to crystallize the photoconductive grade of cadmium sulfide, selenide or telluride, respectively.

The cadmium metal and the alkyl sulfur, selenium or tellurium compounds are preferably reacted in substantially stoichiometric proportions, since it is undesirable in the production of such pure cadmium compounds to have an excess of either the metal or the alkyl compound because of the danger of forming conductors or insulators in the semi-conductor product. For example, various metal impurities such as iron and nickel form impurity centers or traps for the electrons and therefore are undesirable. Even infrared radiation cannot permanently remove these traps. Desired impurities may be added, such as copper, silver, zinc (activators) and chlorine, indium, gallium (coactivators) under controlled conditions in order to have the semi-conductor detect at the desired wave lengths ranges.

The following examples illustrate specific embodiments of the present invention:

Example 1 The preparation of cadmium sulfide of photoconductive single-crystal grade is carried out by first charging 94.14 g. of zone-relned cadmium metal in a boat located in an electric furnace at 700 C. to 800 C. The temperature is maintained at 900 C. in the reaction'area within the quartz tube. The quartz tube is provided with two gas inlet tubes, an exhaust vent and a thermocouple shield. One end of the furnace tube is also provided with a glass window in order to permit observation of conditions within the furnace. The window is located at the exhaust end of the tube which extends outside of the furnace. Adjacent to the reaction zone a crystal growing zone is maintained at about 750 C. to 850 C. in order to permit crystal growth from the vapor phase. One'of the gas inlet tubes is provided with a The second gas inlet tube is utilized for the introduction of hydrogen as a carrier gas at the same flow as the above hydrogen. The dimethyl sulfide is provided in an amount which is a function of the partial pressure of the cadmium vapor. The hydrogen gas is provided in an amount corresponding to approximately two times the atomic proportion of the sulfur. It is found that the cadmium sulfide which crystallizes on the tube is an unusually pure product existing in a single-crystal form and having uniform photoconductive properties.

Example 2 The method of Example 1 is carried out by the reaction of cadmium metal with dimethyl disulfide in order to obtain photoconductive cadmium sulfide as the product.

Example 3 The method of Example 1 is carried out charging cadmium metal as a cadmium source and diethyl disulfide 2,980,500 a y A l as the sulfur source. The product obtained is a photo- 7 conductive grade of cadmium sulfide.

Example 4 The method of Example 1 is carried out utilizing cadmium metal which is reacted with dipropyl disulfideto obtain photoconductive grade of cadmium sulfide as the product.

Example 5 The method of Example 1 is carried out by reacting cadmium metal withdiethyl monosulfide to obtain photoconduotive cadmium sulfide as the product.

Example 6 Themethod of Example 1 iscarried out by the reaction of cadmium metal With dipropyl monosulfide to obtain photoconductive cadmium sulfide as the product.

Example 7 The method of Example 1 is carried out by reacting cadmium metal withmethyl mercaptan to obtain cadmium sulfide of photoconductive grade as the product.

Example 8 The method of Example 1 is carried out by reacting cadmium metal with ethyl mercaptan to obtain cadmium of photoconductive grade as the product.

Example 9 The method of Example 1 is carried out by reacting cadmium metal with propyl mercaptan to obtain photoconductive cadmium sulfide as the product.

Example 10 The method of Example 1 is carried out by reacting cadmium metal with diethyl selenide to obtain cadmium selenide of photoconductive grade as the product.

Example 11 The method of Example 1 is carried out by the reaction of cadmium metal with dipropyl telluride to obtain cadmium telluride as the product of photoconductive grade.

Cadmium sulfide has been recognized as such having photoconductive properties. However, the photoconductive properties of this material are extremely sensitive to the presence of impurities. The prior art forms of cadmium sulfide have therefore been unsatisfactory as photoconductive materials, since it has been impossible by prior art means to prepare sufficiently pure cadmium sulfide in order to achieve uniform, reproducible photo-' conductive properties.

However, it has now been found that the single-crystal form of-cadmium sulfide obtained by the present method is of photoconductive grade and is a more pure product than could be obtained by prior methods and from the source materials of the prior art. The photoconductive cadmium sulfide obtained by the present method, when irradiated by light, produces a current which can operate a relay in an electromechanical system. Furthermore, it has been found that such cadmium sulfide is sensitive not only to light in the visible region, but also to radiation of shorter wave lengths including X-rays as well as alpha, beta and gamma radiation. These properties render the photoconductive cadmium sulfide a useful ma terial in space missiles and vehicles in which radiation of the above-described Wave lengths must be detected and measured. For example, the radiation distribution in outer space may readily be measured by such photoconductive cadmium sulfide used in a detection and re cording/ transmission system. Another field of utility for the photoconductive grade of cadmium sulfide is as radiation detector device and monitor, for example in atomic energy installations. The energy gap (2.43 electron volts at room temperature) makes it possible to manufacture radiation detection and monitoring devices which are direct reading without the use of an electronic amplifier. For example, a monitor device may be made from photoconductive cadmium sulfide and this cell connected directly to an ammeter. This meter may then be calibrated to read directly in terms of the dosage of radiation.

The photoconductive cadmium sulfide of the present invention is also useful in a number of devices based upon the detection and actuating effect of visible radiation. Examples of such devices include burglar alarm systems based upon visible light, door opening systems based upon the incidence of a beam of light such as a headlight upon a garage door supplied with a detector of photoconductive cadmium sulfide, and television remote control devices in which a gun emitting a small amount of visible light is aimed at a detection unit which then responds through a relay system to adjust the particular control of a television set.

What is claimed is:

1. Method for the production of a compound selected from the group consisting of cadmium sulfide, cadmium selenide and cadmium telluride which comprises reacting cadmium metal in the vapor phase with a compound selected from the group consisting of alkyl mercaptans, alkyl sulfides, alkyl disulfides, alkyl selenols, alkyl selenides, alkyl diselenides, alkyl tellurols, alkyl tellurides and alkyl ditellurides, in which each alkyl radical has lessthan 4 carbon atoms, and in which the temperature is maintained within the range of from 800 C. to 1000 C.

2. Method for the production of cadmium sulfide which comprises reacting cadmium metal in the vapor phase with an alkyl sulfide in which each alkyl radical has less than 4 carbon atoms, in which the temperature is maintained Within the range of from 800 to 1,000 C.

3. Method for the production of cadmium sulfide, which comprises reacting cadmium metal in the vapor phase with an alkyl disulfide in which each alkyl radical has less than 4 carbon atoms, in which the temperature is maintained within the range of from 800 C. to 1,000 C.

4. Method for the production of cadmium sulfide, which comprises reacting cadmium metal in the vapor phase with an alkyl mercaptan in which each alkylradical has less than 4 carbon atoms, in which the temperature is maintained within the range of from 800 C. to 1,000 C.

5. Method for the production of cadmium sulfide, which comprises reacting cadmium metal in the vapor phase with dirnethyl sulfide, in which the temperature'is maintained within the range of from 800 C. to l,000 C.

6. Method for the production of cadmium sulfide, which comprises reacting cadmium metal in the vapor phase with diethyl disulfide, in which the temperature is maintained within the range of from 800 C. to 1,000 C. p

7. Method for the production of cadmium sulfide, which comprises reacting cadmium metal in the vapor phase with methyl mercaptan, in which the temperature is maintained within the range of from 800 C. to 1,000 C.

8. Method for the production of a compound selected from the group consisting of cadmium sulfide, cadmium selenide and cadmium telluride which comprises reacting cadmium metal in the vapor phase with a compound selected from the group consisting of alkyl mercaptans, alkyl sulfides, alkyl disulfides, alkyl selenols, alkyl selcnides, alkyl diselenides, alkyl tellurols, alkyl tellurides and alkyl ditellurides, in which each alkyl radical has less than 4 carbon atoms, and in which the temperature is maintained within the range of from 750 C. to 1,200 C.

9. Method for the production of cadmium sulfide, which comprises reacting cadmium metal in the vapor phase with an alkyl sulfide in which each alkyl radical has less than 4 carbon atoms, in which the temperature is maintained within the range of from 750 C. to 1,200 C.

10. Method for the production of cadmium sulfide, which comprises reacting cadmium metal in the vapor phase with an alkyl disulfide in which each alkyl radical has less than 4 carbon atoms, in which the temperature is maintained within the range of from 750 C. to 1,200 C.

11. Method for the production of cadmium sulfide, which comprises reacting cadmium metal in the vapor phase with an alkyl mercaptan in which each alkyl radical has less than 4 carbon atoms, in which the temperature is maintained within the range of from 750 C. to 1,200 C.

12. Method for the production of cadmium sulfide, which comprises reacting cadmium metal in the vapor phase with dimethyl sulfide, in which the temperature is maintained within the range of from 750 C. to 1,200 C.

13. Method for the production of cadmium sulfide, which comprises reacting cadmium metal in the vapor phase with diethyl disulfide, in which the temperature is maintained within the range of from 750 C. to 1,200 C.

14. Method for the production of cadmium sulfide, which comprises reacting cadmium metal in the vapor phase with methyl mercaptan, in which the temperature is maintained within the range of from 750 C. to

References Cited'in the file of this patent Sachanen: 2nd Ed. Conversion of Petroleum (1948), pages 113 to 115 and 394 to 396.

Thorps Dictionary of Applied Chemistry, 4th Ed., vol. 11, page 194.

Sidgwick: Chemical Elements and their Compounds, vol. 1, pages 270-271. 

1. METHOD FOR THE PRODUCTION OF A COMPOUND SELECTED FROM THE GROUP CONSISTING OF CADMIUM SULFIDE, CADMIUM SELENDINE AND CADMIUM TELLURIDE WHICH COMPRISES REACTING CADMIUM METAL IN THE VAPOR PHASE WITH A COMPOUND SELECTED FROM THE GROUP CONSISTING OF ALKYL METCAPTANS, ALKYL SULFIDES, ALKYL DISULFIDES, ALKYL SELENOLS, ALKYL AND ALKYL DITELLURIDES, IN WHICH EACH ALKYL RADICAL HAS LESS THAN 4 CARBON ATOMS, AND IN WHICH THE TEMPERATURE IS MAINTAINED WITHIN THE RANGE OF FROM 800*C. TO 1000*C. 